Is there anyone on the site who has read every SCP? Is having read most if not all of them normal, or have most people not read most of them? I'm new to the community, so I would like to learn more about community standards.

There's not really anything in the way of standards for how many a user reads. The idea of reading SCPs (especially modern ones) is to give new authors an idea of what the community expects and responds well to. Adding a required number to that isn't very helpful.

SCPper shows me that I've voted on 3,209 SCPs and Tales, and I'd say over time I've no-voted about a quarter of the pages I've read, so I'd wager I've read about 4,300 pages. I think I'm on the very high end, though.

I think reading something like 200-300 pages would be sufficient for any practical purpose, though if you enjoy reading SCPs, there's no reason not to read more. It's also not like you can't participate in the community until you've read that many, either.